1. Field of the Invention
The present invention relates to a mobile radio terminal capable of executing adaptive modulation.
2. Description of the Related Art
In a radio communication system employing the adaptive modulation, a terminal measures a radio transmission path quality and transmits it to a base station as CQI (Channel Quality Indication). The base station determines a transmission format (combination of the modulation and the encoding rate) on the basis of the CQI and transmits transmission information over an individual information channel using an appropriate transmission format. The type of the transmission format of the individual information channel is transmitted over the control channel as control information immediately before the transmission of the transmission information (cf., for example, 3GPP TS 25.214 V5.11.0 (2005-06) 6A HS-DSCH-related procedures).
In addition, in a system employing OFDM (Orthogonal Frequency Division Multiplexing), the base station executes transmission simultaneously using a number of sub-carriers. At this time, different terminals can be allocated to the respective sub-carriers. This system is similar to the conventional FDM system in terms of allocating specific frequencies, but greatly different in terms of a feature that each terminal simultaneously receives all the sub-carriers of OFDM, executes a receiving process of the OFDM signal and extracts the sub-carrier to be received (cf., for example, 3GPP TSG RAN1#40bis R1-050249 Downlink Multiple Access Scheme for Evolved UTRA Multiplexing of Common Channel and Shared Data Channel). In this document, the sub-carriers are allocated to the terminals by a unit of grouping.
If the above two conventional techniques are combined, a radio communication system employing the following frequency band allocation control and the applied control of modulation and encoding can be constituted.
In a system of simultaneously receiving a plurality of bands such as the OFDM system, the terminal measures a radio transmission path quality or reception quality of sub-bands determined by dividing the entire measured band, and transmits the measurement result to the base station as CQI information.
The base station selects the transmission format to be applied to the individual information channel on which the transmission information is transmitted, and a sub-carrier group (RB: Resource Block) to be used, on the basis of the CQI information transmitted from each of the terminals and the other information saved in the base station. The information of the transmission format applied to the individual information channel and the information of the RB to be used are transmitted to the terminal before the individual information channel is transmitted over the control channel.
In other words, the CQI of each sub-group is transmitted from the terminal, and the information of the transmission format of the individual information channel and the information of the RB to be used are transmitted over the control channel from the base station.
For this reason, the terminal receives all the control channels, detects the control channel on which its own terminal ID is recorded, discriminates the RB to be used over the individual information channel, on the basis of the information on the detected control channel, discriminates the applied transmission format and sets the receiving function, and receives the individual information channel transmitted from the base station.
Incidentally, in some systems, the terminal transmits a Wideband quality measurement value (hereinafter called Wideband CQI) and a quality measurement value (hereinafter called Subband CQI) of Subbands obtained by dividing the Wideband, as the CQI information to be transmitted to the base station (cf., for example, 3GPP TSG RAN1#51 R1-075064).
In such a system, the Wideband CQI is notified as the CQI information, by a CQI report value representing its quality at, for example, 32 stages while the Subband CQI is notified as the CQI information, by a Diff value representing a differential value of, for example, 8 stages about the CQI report value of the Wideband including the Subband.
As for the CQI information of this system, however, if the CQI report value is close to the upper limit or lower limit, the represented range of the Subband CQI based on the Diff value is made narrow and the bit sequence required for the transmission of the Diff value cannot be efficiently used.